The present invention relates to a holographic device for formation of at least first and second colored, angularly separated light beams, and more particularly, to a device for formation of such beams with the same planar polarization and having first and second predetermined spectral compositions respectively. Still more particularly, the invention relates to a video image projector incorporating such a device.
A device is known from the patent U.S. Pat. No. 5,161,042, which is based on the use of dichroic mirrors for angularly separating three light beams of different colors illuminating a matrix screen of liquid crystal cells through a grid of micro-lenses, for the projection of a video image displayed on this screen. The three beams are separated angularly in the same plane and are therefore suitable if the three liquid crystal cells which define a pixel of the image registered on the screen are aligned. They are not suitable if they are arranged at the apexes of a triangle, according to the configuration called xe2x80x9cxcex94xe2x80x9d, for example. Furthermore, the three beams obtained are not polarized linearly, and it is therefore not possible to do without the two crossed polarizers which ordinarily equip a liquid crystal matrix screen and which have the disadvantage of absorbing a substantial quantity of the luminous energy.
Also known from the international patent application WO-A-92/09915 is a holographic device for illumination of such a screen by suitably colored and polarized light beams, formed by three distinct sets of prisms and holograms, which make the device bulky. The present invention aims to produce a holographic device for formation of colored and polarized light beams which are angularly separated, for the projection of a video image displayed on a matrix screen of liquid crystal cells, whose space requirement is as small as possible.
The present invention also aims to produce such a device making it possible to angularly separate three such beams according to three non-coplanar directions, for the illumination of a liquid crystal screen in which the three cells defining a pixel of the image are not arranged in a line.
The present invention furthermore aims to produce such a device which has a high luminous yield and good compactness.
These aims of the invention are achieved, as are others which will appear upon reading of the following description, with a holographic device for formation of at least first and second angularly separated light beams, with the same planar polarization, and having first and second predetermined spectral compositions respectively, this device being remarkable in that it has first and second roughly coplanar and superposed holograms, which are illuminated by the same nonpolarized light beam with at least said first and second spectral compositions, said holograms each being recorded in a manner such that in its middle, the axis of the diffracted beam is perpendicular to the direction of the incident beam, said diffracted beams forming said first and second angularly separated light beams with the same polarization, and having first and second predetermined spectral compositions respectively.
As will be seen in detail subsequently, the light beams thus formed are produced by the same optical assembly, with a reduced space requirement.
According to one embodiment of the invention, the holograms are squeezed between two attached adjacent prisms, made of optical material. They are recorded in a material with a refractive index roughly equal to that of the material constituting said prisms. They are illuminated by a beam of light with a 45xc2x0 incidence, the axes of the beams diffracted by these holograms converging on the axis of the incident beam hitting the holograms and being in a plane perpendicular to this axis.
According to a first variant, the device includes a third hologram roughly coplanar with the other two, recorded so as to form a diffracted beam of light with planar polarization with a third predetermined spectral composition, whose axis is coplanar and converging with those of the other two diffracted beams. As will be seen subsequently, such a device is suitable for illuminating matrix screens of liquid crystal cells in which the cells of the triplets, each of which defines a pixel of the displayed image, are aligned.
According to another variant of the device according to the invention, a third hologram is included which is not coplanar with the other two, recorded in a material with a different index from the index of the other two, and arranged to form a third diffracted beam whose axis is not coplanar with the axes of the beams diffracted by the first and second holograms, said third beam being polarized linearly like the other two and having a third predetermined spectral composition. In this variant, the device includes a third cuneiform prism inserted between the other two prisms, the first and second holograms being squeezed between a face of this third prism and a face of one of the other prisms, while the third hologram is squeezed between another face of this third prism and a face of the other of the other prisms, the planes of the holograms converging on an edge common to the three prisms. As will be seen subsequently, this variant is suitable for the illumination of a matrix screen of liquid crystal cells in which the cells of the triplets of cells, each of which defined a pixel of the image to be displayed, are not aligned and are arranged, for example, in xcex94 configuration